Travelling wave tube



mwm

7, W56 N. W. ROBINSON TRAVELLING WAVE TUBE Filed Aug. 28, 1950 INVENTOR.

NORMAN, WEIGHT ROBINSON AGENT TRAVELLING WAVE TUBE Norman WrightRobinson, 'Salfords, near Redhill, England, assignor to HartfordNational Bank and Trust Company, Hartford, Conn., as trustee ApplicationAugust 28, 1950, Serial No. 181,754

Claims priority, application Great Britain September 1, 1949 6 Claims.(Cl. SIS-3.5)

This invention relates to travelling wave tubes.

Known travelling wave tubes comprise a long, usually tightly wound wirehelix and an electron gun adapted to direct a beam of electrons in adirection parallel to the axis of and within the helix. Anelectro-magnetic wave may be impressed on the helix at the end adjacentthe cathode and caused to pass along the helix travelling in the samesense as the electron beam. The operation of the travelling wave tubedepends on the fact that the electro-magnetic wave and the electronstravel along the travelling wave tube with about the same velocity. Theelectrons, entering the helix with a velocity slightly greater than theaxial velocity of the electro-magnetic wave, tend to become bunched asthey travel through the helix. A power gain for propagation in thedirection of electron motion may thus be obtained, energy beingabstracted from the tube at about the end of the helix remote from theelectron gun. The final anode of the electron gun usually consists of acopper-nickel tube of the same internal and external diameter as thehelix, and electrical contact between the metal tube and the helix ismade by welding. A similar conection to a second metal tube is requiredto be made at the output end of the helix.

The helix may be supported along the whole of its length, together withthe final anode and the second metal tube at the output end of thehelix, by quartz or glass tubing of circular internal cross section. Inthis case, the quartz or glass tubing has to be slightly larger ininternal diameter than the outer diameter of the helix in order toaccommodate the extra thickness at the welds. The axis of the helix,therefore, tends to depart from the linear. A further disadvantage ofthe use of quartz or glass tubing of circular cross section is thatthere is a region of high dielectric constant in close contact with thehelix, which gives rise to distortion of the longitudinal field in thecentre of the helix. irregularities in the thickness of the quartz orglass also tend to cause distortion.

Alternatively, the helix may be spaced from the glass or quartz tubingand supported at intervals by glass or quartz dimples providedinternally of the quartz or glass tubing. The main disadavntage of thisconstruction of travelling wave tube is that, since the mechanicalstrength of the helix is small, the helix tends to sag between adjacentpoints of support so that the axis of the helix is not linear.

The invention provides a travelling wave tube in which the axis of thehelix is substantially linear, the helix is in contact with a materialof high dielectric constant at only a few points in each turn and theeffects of irregularities in the material of high dielectric constantare minimised.

According to the invention, a travelling wave tube comprises a long wirehelix, means to generate an electron beam and to direct said beam in adirection parallel to the axis of and within the helix and a quartz,glass or ceramic tube supporting said helix, the internal cross sectionof the quartz, glass or ceramic tube being a regu- States Patent2,758,241 Patented Aug. 7, 1956 lar figure having three or more sides,the outer circumference of the cross section of the helix forming theinscribed circle in the regular figure. The sides of the regular figuremay be linear or may be concave or convex.

Preferably the cross section of the quartz, glass or ceramic tube is athree sided regular figure. Above a limit of ten sides little advantageis gained since the cross section is then approximately a circle.

In a travelling wave tube according to the invention, the welded jointsbetween the helix and the two tubes, which may be copper nickel tubes,one at each end of the helix, may be accommodated in the corners of theglass tubing externally of the circle inscribed in the regular figure.

Two examples will now be given of the manner in which suitable quartz,glass or ceramic tube may be manufactured.

Starting with quartz or glass tubing of substantially circular internalcross section having at any point along its length an internalcircumference not greater than the internal perimeter required in thefinished tubing, a suit ably shaped mandrel is caused to travel throughthe tubing under the action of a substantially constant driving force,the tubing being heated at or near the zone of engagement with themandrel so that the heated zone travels along the tubing as the mandrelprogresses. The mandrel tapers towards its leading end to a size smallerthan the original bore of the quartz or glass tubing. The internal crosssection of the finished tubing may, of course, be obtained by way of anumber of intermediate stages. This manner of manufacture is suited tothe production of quartz or glass tubes the cross section of which is aregular figure having four or more sides. With a three sided figure wearof the mandrel is more pronounced. In any case the bore of the tubeshould be such that the helix is regular to about /2%. Wear of themandrel at the corners is not objectionable; in this case a regularfigure having curved instead of linear sides is produced. A typicalhelix for use in a travelling wave amplifying tube comprising such aquartz or glass tube and operating in the 7,000-10,000 mc./s. region isapproximately 10" long and 2.5 mm. outside diameter, the helix beingwound with 48 turns per inch of wire 0.25 mm. in diameter.

Starting with a solid strip, being a right cylinder of rectangular crosssection, of quartz or radio frequency ceramic, such for example as thatavailable commercially under the name Frequentite, a V is machined atabout the centre of one longitudinal surface of the strip. A secondstrip of quartz or radio frequency ceramic is secured to the first. Thewalls of the tube so provided may be thick compared with the dimensionsof the cross section of the bore. The very regular bore of triangularcross section so produced is such that a helix approximately 10 long and1 mm. outside diameter may be used, the helix being wound with turns perinch of wire 0.1 mm. in diameter.

In order that the nature of the invention may be more readily understoodreference will now be had to the accompanying diagrammatic drawing, inwhich:

Fig. 1 is a longitudinal section of a first travelling wave tube,

Fig. 2 is a section taken on the line Il'II of Fig. 1.

Fig. 3 is a detail view showing the connection between one end of thehelix and the final anode of the electron gun, and

Fig. 4 is a cross section of a second travelling wave tube.

Referring now to Fig. 1, the travelling wave tube com- 0 prises anelectron gun 1, including a cathode 21, grid 22,

accelerating member 23 and final anode 2, and a Wire helix 3 connectedat one end to the final anode 2 and at the other end to a copper-nickeltube 4 at the output end. The whole is enclosed in an outer tube 5, forexample of glass, having a collector electrode 6 at the end of the tuberemote from the electron gun 1, the helix being supported in position bya glass or quartz tube 7 of square inner and outer cross section. Thehelix is connected electrically by a weld 8 to the final anode 2 and bya weld 9 to the metal tube 4 at the output end. An attenuator 10consisting of a colloidal graphite deposit on the inner surface of theglass or quartz tube, is provided in known manner to prevent selfoscillation when the travelling wave tube is in use. A choke 11 isprovided at each end of the tube.

Fig. 2 shows the outer tube 5, the quartz or glass tube of square crosssection 7 and the helix 3. It will be seen that the outer circumferenceof the cross section of the helix 3 forms the inscribed circle in theinner square and makes contact with the tube 7 at four points only perturn.

Fig. 3 shows the final anode 2 held in position within the tube 7 by ashim 12, the helix 3 and the joint 8. The joint 8 at any point along itslength is accommodated in one angle of the inner square externally ofits inscribed circle.

Fig. 4 shows the outer tube 5 and the helix 3 of a second travellingwave tube. The helix is held in position between two strips 13 and 14 ofquartz or Frequentite. In the strip 14 a V shaped slot has been machinedsuch that in section the slot 15 and the adjacent longitudinal surfaceof the strip 13 form an equilateral triangle, the outer circumference ofthe cross section of the helix 3 forming the inscribed circle in thetriangle.

What I claim is:

1. An electric discharge tube of the travelling wave type, comprising anelectron gun including an anode structure for directing a beam ofelectrons along a given axis, a hollow conductive helix having a givenlength and having a given axis parallel to the axis of and surroundingthe electron-beam and connected at one end to the anode, a conductingtube surrounding the axis of the electron beam connected to the otherend of the helix, a collector electrode adjacent the end of the tuberemote from the helix, and a tube of electrically insulating materialhaving a length at least as great as said given length enclosing andsupporting the entire helix, said tube having a regular polygon internalcross-section, whereby each turn of the helix is only engaged at a fewspaced points.

2. An electric discharge tube of the travelling wave type, comprising anelectron gun including an anode structure for directing a beam ofelectrons along a given axis, a hollow conductive helix having a givenlength and having a given axis parallel to the axis of and surroundingthe electron beam and connected at one end to the anode, a conductingtube surrounding the axis of the electron beam connected to the otherend of the helix, a collector electrode adjacent the end of the tuberemote from the helix, and a tube of vitreous material having a lengthat least as great as said given length enclosing and supporting theentire helix, said tube having an internal cross-section of a regularpolygon of at least three and at most ten sides, whereby each turn ofthe helix is only engaged at a few spaced points.

3. An electric discharge tube of the travelling wave type, comprising anelectron gun including an anode structure for directing a beam ofelectrons along a given axis, a hollow conductive helix having a givenlength and having a given axis parallel to the axis of and surroundingthe electron-beam and connected at one end to the anode, a conductingtube surrounding the axis of the electron beam connected to the otherend of the helix, a collector electrode adjacent the end of the tuberemote from the helix, and a tube of vitreous material having a lengthat least as great as said given length enclosing and supporting thehelix, said tube having a triangular internal cross-section, wherebyeach turn of the helix is only engaged at a few spaced points.

4. An electric discharge tube of the travelling wave type, comprising anelectron gun including an anode structure for directing a beam ofelectrons along a given axis, a hollow conductive helix having a givenlength and having a given axis parallel to the axis of and surroundingthe electron beam and connected at one end to the anode, a conductingtube surrounding the axis of the electron beam connected to the otherend of the helix, a collector electrode adjacent the end of the tuberemote from the helix, and a tube of vitreous material having a lengthat least as great as said given length enclosing and supporting thehelix, said tube comprising a rod having a V-shaped recess and a flatplate covering the recess thereby forming a channel having acrosssection in the form of an equilateral triangle for accommodatingand supporting the conductive helix, whereby each turn of the helix isonly engaged at a few spaced points.

5. An electric discharge tube of the travelling wave type, comprising anelectron gun including an anode structure for directing a beam ofelectrons along a given axis, a hollow conductive helix having a givenlength and having a given axis parallel to the axis of and surroundingthe electron-bearn and connected at one end to the anode by a weldedjoint, a conducting tube surrounding the axis of the electron beamconnected to the other end of the helix by a welded joint, a collectorelectrode adjacent the end of the tube remote from the helix, and a tubeof vitreous material having a length at least as great as said givenlength enclosing and supporting the helix, said tube having a regularpolygon internal crosssection, whereby each turn of the helix is onlyengaged at a few spaced points, the welded joints being accommodatedwithin the angles formed by the sides of the polygon.

6. An electric discharge tube of the travelling wave type, comprising anelectron gun including an anode structure for directing a beam ofelectrons along a given axis, a hollow conductive helix having a givenlength and having a given axis parallel to the axis of and surroundingthe electron beam and connected at one end to the anode, a conductingtube surrounding the axis of the electronbeam connected to the other endof the helix, a collector electrode adjacent the end of the tube remotefrom the helix, and a tube of vitreous material having a length at leastas great as said given length enclosing and supporting the helix, saidtube having an internal rectangular cross-section, whereby each turn ofthe helix is only engaged at a few spaced points.

References Cited in the file of this patent UNITED STATES PATENTS1,901,703 Crowley Mar. 14, 1933 2,064,469 Haeif Dec. 15, 1936 2,300,052Lindenblad Oct. 27, 1942 2,575,383 Field Nov. 20, 1951 2,611,101Wallauschek Sept. 16, 1952

